1. Field of the Invention
The present invention relates to handsfree telephones having multiple channels.
2. Description of the Prior Art
Handsfree telephone technology of the known prior art utilizes voice switching or gain variation techniques to overcome the limitations imposed by the acoustic path between the microphone and loudspeaker and the imperfect sidetone cancellation provided by the hybrid. Voice switching is accomplished by the use of analog switches or field effect transistors (FET's) to alternately turn the communication channels on and off. The switches are controlled by the transmit signal, receive signal, or a combination of both signals in a comparator circuit. This known process of switching the communication channels in the transmission path on and off creates an annoying side affect known as clipping. Clipping occurs at the leading edge of the switched waveform and usually causes a portion of the spoken text to be truncated.
The variable gain method, also known in the prior art, although causing less noticeable clipping, still can create "pumping" or "breathing" sounds as the gain moves up and down and can be equally disturbing to the handsfree telephone user, as well as to the distant telephone subscriber. For these reasons, most telephone subscribers experienced in the use of handsfree telephony would prefer a full duplex subset where no switching or gain changing ever occurs. However, current state-of-the-art full duplex handsfree telephones suffer from low transmission levels dictated by the lack of acoustical isolation. Digital signal processing may overcome these limitations in time.